A big part of political inertia is building and maintaining pressure on your opponents. Labour are currently trying to strip the Tories of their base by politically marching rightwards. The Tories on the otherhand are continuing their age old policy of stripping Labour of their working class base by duping the poor to voting against their own interests and collective responsibilities by inflaming their base instincts through the medium of tabloid-esque campaigns.

The London population firmly leans to the left so Labor only have to field a competent scandal-free (though of course scandals can always be manufactured) candidate for a shoe-in. Even with Khan's milquetoast record the Tories had very little hope of replacing him.

So the political calculus was find a fall-guy (not one of their big guns) to take the L but bombastically push the type of toxic shite that the base of politically illiterate narcissists they have cultivated need to stay angry. Even though much of the issues were local to London the amount of extra airtime the London Mayoral race gets makes it a useful vehicle for more realistic target constituencies outside of the M25.

Take the ULEZ or LTN. Both are sensible approaches to reducing air pollution and improving quality of life in London. And most Londoners agree because they have to breath the air that's available. However outside of London this issue becomes more about car owner's rights as narcissists care less about how their journey into and around London effects the people that live there than their own convenience. So the Tories have set their stall up against these modest policies. They have been supporting and catalysing the tiny fanatical groups you occasionally see out protesting against them in London. These small groups make a lot of 'noise' in the news and online for little or no cost. That creates pressure on that part of social-norms that limit the perceived degree of acceptable selfishness and that's a win for the Tories.